1. Technical Field
This disclosure relates to an input protection circuit.
2. Related Art
Some electronic devices such as a voltage measuring device that measures an input voltage with high accuracy include input protection circuits. The input protection circuit protects an internal circuit when an input terminal receives an excessive voltage. FIG. 8 illustrates an exemplary known input protection circuit in an electronic device. An input protection circuit 2 protects a voltage measuring circuit 1 against the excessive input voltage Vin. The voltage measuring circuit 1 receives a positive power supply voltage VDD that is a power supply voltage at a positive electrode side and a negative power supply voltage VSS that is a power supply voltage at a negative electrode side. The voltage measuring circuit 1 measures an analog voltage with high accuracy.
The input protection circuit 2 includes, between an input terminal 3 to which the input voltage Vin is applied and the voltage measuring circuit 1, a Negative channel Metal-Oxide-Semiconductor Field-Effect Transistor (NMOSFET) 4 and Positive channel Metal-Oxide-Semiconductor Field-Effect Transistor (PMOSFET) 5 that are coupled in series. The NMOSFET 4 has a gate terminal to which the positive power supply voltage VDD is applied. The PMOSFET 5 has a gate terminal to which the negative power supply voltage VSS is applied.
The NMOSFET 4 has a drain coupled to the input terminal 3. Its source is coupled to a drain of the PMOSFET 5. The PMOSFET 5 has a source coupled to the voltage measuring circuit 1.
In the case where the input voltage Vin is within a power supply voltage range (VDD to VSS) of the voltage measuring circuit 1, gate-source voltages of the NMOSFET 4 and the PMOSFET 5 change within a range of 0V to VDD+|VSS| corresponding to a value of the input voltage Vin. At this time, the NMOSFET 4 and the PMOSFET 5 are both in the on-state. Accordingly, the input voltage Vin is input to the voltage measuring circuit 1 as Vic through the NMOSFET 4 and the PMOSFET 5. Here, for ease of explanation, the threshold voltages of the MOSFETs 4 and 5 are assumed to be zero V.
In the case where the input voltage Vin becomes higher than the positive power supply voltage VDD of the voltage measuring circuit 1, the NMOSFET 4 becomes off-state. Thus, the input voltage Vin is not input to the voltage measuring circuit 1. In the case where the input voltage Vin becomes larger than the negative power supply voltage VSS of the voltage measuring circuit 1 with respect to a negative electric potential side, the PMOSFET 5 becomes off-state. Thus, the input voltage Vin is not input to the voltage measuring circuit 1. This switching operation protects the voltage measuring circuit 1 against the excessive input voltage. This type of known technique is described in, for example, U.S. Pat. No. 5,389,811.